A conventional matrix-addressed smectic cell has a first set of electrodes, row electrodes, which are intersected by a second set, column electrodes, that extend across the first set. In this way the position of a pixel, that is the area of intersection of any individual row electrode with any individual column electrode, is uniquely defined by its row number coupled with its column number.
Although in conventional usage the term `row` is normally reserved for electrodes that extend from side to side of the display area, and `column` for those that extend from top to bottom, for the purposes of this specification the terms `row` and `column` are to be understood as not restrictive as to the direction in which they extend. Thus, for instance, any reference to a row of characters will normally refer to a set of characters extending in a single line across the display in the manner conventionally employed for setting out consecutive alphanumeric characters or arranged in a line extending up and down the display in the manner conventionally employed for setting out a sequence of Chinese ideograms.
A conventional method for entering data into a matrix addressed liquid crystal cell is to write the data a line at a time by applying a strobing pulse of voltage V.sub.s to each row electrode in turn while the column electrodes are fed in parallel with data pulses of voltage .+-.V.sub.D. The unselected row electrodes, that is the electrodes of all the rows other than that currently receiving the strobing voltage V.sub.S, are held at zero volts. Thus the potential developed across a pixel while its row is being strobed is (V.sub.S +V.sub.D) or (V.sub.S -V.sub.D) according to whether it is to be written into a `1` state or a `0` state. When other rows are being strobed the potential developed across the pixel is V.sub.D.
A smectic liquid crystal display exhibits storage and its response to a drive signal can be cumulative. If a pixel is switched into a particular state by a pulse of a particular voltage and duration, it will in general be possible to switch that pixel to the same extent in a shorter time by using a pulse or larger voltage. Conversely the use of a lower voltage will require a pulse of longer duration. In any particular instance there will be a threshold voltage value V.sub.T which requires a pulse of infinite duration to achieve the requisite switching, or partial switching. Although V.sub.T has been defined in terms of a mathematical limit, it is nevertheless a real physical quantity that may be readily determined in the laboratory to any desired degree of accuracy, since a voltage even slightly above V.sub.T will achieve switching in a finite time and the limiting value of V.sub.T may be readily extrapolated from several finite measurements.
Clearly if V.sub.D &lt;V.sub.T and (V.sub.S -V.sub.D)&lt;V.sub.T unselected elements are never exposed to a voltage equal to or greater than V.sub.T, and hence no amount of switching on of selected elements will ever give rise to the spurious switching on of any unselected element. However, a corollary of this is that the switching voltage (V.sub.S +V.sub.D) to which selected elements are exposed is limited to a value which must be less than 3V.sub.T.
When data is being entered into a smectic display in a mode that involves the entry of the data in complete lines, a complete line at a time, the unselected pixels of that line see (V.sub.S -V.sub.D) for the same duration as the selected pixels see (V.sub.S +V.sub.D). The cell exhibits storage, and hence there is no need to refresh that line, which therefore will remain until it needs to be updated. When the line does need updating it will be cleared before entry of the revised data. It is seen therefore, that an unselected element may see an indeterminite number of pulses of voltage V.sub.D while other rows are being addressed, but it can expect to see only one pulse of voltage (V.sub.S -V.sub.D). Clearly, for absolute safety, V.sub.D must be kept less than V.sub.T since there is no certain limit to the cumulative exposure of the element to this voltage, but on the other hand its exposure to (V.sub.S -V.sub.D) is for a strictly limited duration, the duration required to switch a selected pixel with the voltage (V.sub.S +V.sub.D). It follows therefore, that to restrict the value of V.sub.S to a value which will satisfy the relationship (V.sub.S -V.sub.D)&lt;V.sub.T is to impose an unnecessarily severe requirement upon the system. V.sub.S can be significantly increased to produce a correspondingly significant saving in the required duration of the pulses. For this reason it is generally appropriate, whenever data is to be entered into the display in a mode where an entire row of pixels is entered with a single strobing pulse, to use a large strobing voltage V.sub.S &gt;2V.sub. T in order to increase the rate at which lines can be entered. This mode of data entry is which an entire row of pixels is entered with a single pulse will be termed `whole row entry mode`.
For some applications however, it may not be desirable or even possible to wait for the data of an entire row before beginning to display parts of that row. A particular example of such an application is when the display is required to display each character of a line of alphanumeric characters as it is entered into the system for instance directly from a keyboard. Each of these characters of a character line will need to be entered to the right of its predecessor. If each character is formed by a matrix of `x` by `y` pixels, and the top left-hand pixel of the first character of a line has the co-ordinates (r,s), then rows `s` to `s+y-1` will need to be strobed for entry of that character. The data for entry of that character will be confined to columns `r` to `r+x-1`. All the other columns will be unselected columns. Entry of the next character will involve a repetition of the strobing of rows `s` to `s+y-1`, but in this instance the data entry is confined to columns `r+x` to `r+2x-1`, all other columns being unselected. Therefore, upon entry of the second character all pixels of rows `s` to `s+y-1` that have a column co-ordinate of `r+2x` or greater will receive a second unselected pixel pulse of voltage (V.sub.S -V.sub.D). If `whole row entry mode` strobing pulse voltage levels are used, the entry of a succession of different segments of a row is liable soon to run into the problem that an accumulation of (V.sub.S -V.sub.D) pulses will be sufficient to cause a spurious writing of unselected elements. A data entry mode that involves the entry of a succession of different segments of a row will be termed `segmented row entry mode`.
One way of overcoming this problem of the spurious writing of unselected elements in segmented row entry mode is to arrange to erase the row between each consecutive data entry into that row. Clearly this requires that the pre-existing data of that row is at least temporarily stored elsewhere so that it is not lost upon erasure, but is available for re-entry with the data pertaining to the entry of the next character. The resulting temporary loss of display of a row immediately prior to the entry of a fresh segment might be acceptable in some circumstances if it were not for the fact that it is found that the temporary erasure is associated with a temporary brightening of the background during the erasure. The result is that this approach to solving the problem of spurious writing of unselected elements when using segmented row entry mode produces its own problem namely that the row `flashes` in a most distracting way.